1) have a bigger takeback...you don't need to "scratch your back" as such, but i feel you need to take the racket back lower behind your back, else no power is generated.

2) always try to contact the ball as high as you can

You seem to be "late" sometimes...causing you to bend your wrist at contact. You want your wrist to extend, contact and pronate after contact. Much like a whip changes direction at highest speed...or when you hit your finger with a hammer, you "whip" the finger? Its a loose action. So basically on contact that "whip" speed is what you want - the maximum speed is then, and not before or after contact.